NASA researchers say they have have spotted iceberg-like “hills” floating on a sea of frozen nitrogen on Pluto.

The hills, which are seen in the “Sputnik Planum” plane inside the so-called “heart,” are from one to several kilometers across, according to photos beamed back to Earth from the New Horizons probe.

NASA believes the hills are made of water ice because water is less dense than nitrogen.

The hills, NASA says, move much like Earth’s icebergs and are “likely fragments of the rugged uplands that have broken away and are being carried by the nitrogen glaciers into Sputnik Planum.”

Unlike Earth’s icebergs, which move relatively rapidly, the icebergs on Pluto move very slowly and cycle around the Sputnik Planum, where they are eventually pushed to the edges over millions of years.

The images also show an area called Challenger Colles, named after the space shuttle Challenger, which blew up 30 years ago, has a “large” accumulation of the hills. They think the hills may have been “beached” in shallow nitrogen.

“They are yet another example of Pluto’s fascinating and abundant geological activity,” said NASA in a press release.